He means they sit completely still, as still as statues.
They sit on logs, so the statues they most resemble are statues of people mounted on horseback (equestrian statues) - the horse being the equivalent of the log.
This is the ‘torpor’ he describes elsewhere in the para:
With respect to the torpor supposed to follow, or rather (if we were to credit the numerous pictures of Turkish opium-eaters) to accompany the practice of opium-eating, I deny that also. Certainly, opium is classed under the head of narcotics; and some such effect it may produce in the end: but the primary effects of opium are always, and in the highest degree, to excite and stimulate the system: this first stage of its action always lasted with me, during my noviciate, for upwards of eight hours; so that it must be the fault of the opium-eater himself if he does not so time his exhibition of the dose (to speak medically) as that the whole weight of its narcotic influence may descend upon his sleep. Turkish opium-eaters, it seems, are absurd enough to sit, like so many equestrian statues, on logs of wood as stupid as themselves. But that the reader may judge of the degree in which opium is likely to stupify the faculties of an Englishman, I shall (by way of treating the question illustratively, rather than argumentively) describe the way in which I myself often passed an opium evening in London, during the period between 1804-1812. It will be seen, that at least opium did not move me to seek solitude, and much less to seek inactivity, or the torpid state of self- involution ascribed to the Turks. I give this account at the risk of being pronounced a crazy enthusiast or visionary: but I regard /that/ little: I must desire my reader to bear in mind, that I was a hard student, and at severe studies for all the rest of my time: and certainly had a right occasionally to relaxations as well as the other people: these, however, I allowed myself but seldom.